1. Field of the Invention
The present invention relates to a bed positioning system for radiation therapy.
2. Description of the Related Art
During radiation therapy, X-ray transparent images of the subject (patient) on a bed are captured and the bed is positioned according to the result of comparison between the captured images and a reference image in the treatment plan so that the position of the subject coincides with a planned position (position specified in the treatment plan).
In high-precision radiation therapy such as particle beam therapy and X-ray IMRT (Intensity Modulated Radiation Therapy), dose distribution concentrating at the target can be realized. Therefore, the positioning process is also required to position the target in the subject at the planned position with high accuracy.
For the positioning at the time of treatment (radiation therapy), an X-ray transparent image or an X-ray CT (Computed Tomography) image can be used. The X-ray transparent image is a two-dimensional image in which the positions of structures in high X-ray contrast (mainly, bone structure) can be clearly recognized and checked. The X-ray transparent image is shot in two directions orthogonal to each other and used for the positioning based on the bone structure, etc. in the subject. The X-ray CT image is a three-dimensional image in which soft tissues can also be recognized and checked. By use of the X-ray CT image, the position of the target of the radiation therapy is checked and the positioning of the subject is carried out. Since the two types of images are used selectively depending on the type of the treatment target in the subject and the method of the treatment, it is desirable that the two image capturing methods can be executed selectively in the same system.
The positioning is performed by using X-ray transparent images as follows. A digitally reconstructed radiograph (DRR) is generated from a treatment plan CT image by means of numerical simulation based on the relative positional relationship between the subject and the therapy system. An X-ray transparent image is previously captured at the position exactly according to the treatment plan. The digitally reconstructed radiograph (DRR) or the X-ray transparent image obtained is made to coincide with the subject position captured in the transparent image at the time of the treatment (2D/2D registration). Alternatively, digitally reconstructed radiographs (DRRs) according to various relative positional relationships between the subject and the therapy system are calculated from the treatment plan CT image and the positioning is carried out by calculating the deviation of the subject based on comparison with the transparent image at the time of the treatment (3D/2D registration).
In the positioning by use of a CT image, the cone-beam CT is mainly used. The radiation therapy system is generally equipped with a rotatable therapeutic radiation irradiation system for applying the therapeutic radiation to the subject (target) from any direction. By adding an X-ray image capturing system to the therapeutic radiation irradiation system, rotating image capturing is performed and the cone-beam CT image capturing function is implemented. A soft tissue position in the subject captured by the CT image capturing is made to coincide with the planned position, or the positioning is performed by using a bone structure or the like and whether the soft tissue position is at the correct position or not is checked. The X-ray image capturing system provided for the radiation therapy system is generally used also for the positioning by use of X-ray transparent images, enabling both types of positioning with a small number of devices.
A background technology for removing a structure other than the subject from a transparent image has been described in Japanese Patent No. 4344825. This patent literature describes a system for positioning a patient by using a digitally reconstructed radiograph (DRR) and an irradiation field confirmation image (LG: Linac Graphy), wherein the digitally reconstructed radiograph and the LG are registered with each other, the edge of a collimator in the LG is removed based on shape data of the collimator, and the deviation between the digitally reconstructed radiograph and the LG is detected by using the image obtained by the removal of the collimator edge.